Collectors update

[Brian Goetz]

> Just throwing this out there: Have we already strongly considered the
> heretical idea of just naming it reduce()?

Yes.  There are a few reasons we leaned towards a different name here.

1.  (Pedagogical): The *mutable* reduce-like/fold-like methods have a 
different feel than the more functional analogues.

2.  (Practical): Overloading methods that take the same-shape lambdas 
causes use-site ambiguity problems.  Since there are reduce forms that 
look suspiciously like collect forms, this was an issue.

> It's distinguished from the others by the type it takes (and that's not
> a lambda-able type so no worries on that front).

No, because there's the multi-arg collect() form.  Which is not only 
useful (collect(ArrayList::new, ArrayList::add, ArrayList::addAll)) but 
also usefully pedagogically (in that the single-arg collect then becomes 
clearly a generalization of the three-arg one.)

> If we were uncomfortable calling the type accepted Reducer, perhaps
> there's... MutatingReducer.  Bleh.

We had that once!  MutableReduce, it was.  Though mutableReduceUnordered 
is a mouthful.